The present invention relates generally to a line monitoring system employed in a lightwave communication system, and more particularly to a line monitoring system that accurately determines the gain profile of an optical amplifier.
Commercial lightwave systems use optical fibers to carry large amounts of multiplexed digital data over long distances from a transmit terminal to a receive terminal. The maximum distance that the data can be transmitted in the fiber without amplification or regeneration is limited by the loss and dispersion associated with the optical fiber. To transmit optical signals over long distances, the lightwave systems may include a number of repeaters periodically located along the fiber route from the transmit terminal to the receive terminal. Each repeater boosts the weak received signal to compensate for the transmission losses which occurred from the previous repeater. Prior to the widespread availability of efficient optical amplifiers, many systems converted the optical signals into electrical signals for amplification by conventional electrical amplifiers. The amplified electrical signals were then reconverted to the optical domain, for further distribution along the optical communication path. The advent of reliable and low cost optical amplifiers has obviated the need to convert signals into the electrical domain for amplification.
Optical amplifiers, such as rare earth doped optical fiber amplifiers, require a source of pump energy. In a rare earth doped optical fiber amplifier, for example, a dedicated pump laser is coupled to the doped fiber for exciting the active medium (rare earth element) within the amplifier. At the same time, a communication signal is passed through the doped fiber. The doped fiber exhibits gain at the wavelength of the communication signal, providing the desired amplification. If the doped optical fiber is doped with erbium, for example, pump energy may be provided at a wavelength of 1480 nm or 980 nm.
Optical communication systems often employ a line monitoring system (LMS) to monitor the performance of the repeaters. The line monitoring system includes line monitoring equipment located in the terminal and high-loss loop-back paths (HLLB) in the repeaters, which couple a portion of the optical signal back to the transmitting terminal along the opposite-going transmission path. The LME""s data facilitates routine analysis to detect changes in system performance over time. In particular, useful information that may be monitored includes degradations in pump power, loss in the amplifier output stage, loss in the transmission span, and the gain profile of the amplifier (i.e., the shape of the gain as a function of wavelength across the amplifier bandwidth).
In a known line monitoring system, the gain profile is determined by transmitting an amplitude modulated (AM) probe tone at a particular optical wavelength and measuring the gain imparted to the tone. By sweeping the tone""s wavelength across the optical bandwidth of the amplifier, the gain profile can be readily determined. However, the gain profile that results from this series of measurements is distorted because the tone generates cross gain modulation at wavelengths other than the tone wavelength. The cross gain modulation is caused by Raman gain and the time dynamics of the erbium doped fiber. The cross modulation caused by the Raman gain arises because the AM modulated tone in effect serves as a time-varying pump source that generates Raman gain at longer wavelengths. The cross modulation caused by the time dynamics of the fiber is also a result of the time-varying nature of the probe tone. As a result of the cross modulation the correlation between the detected AM level and the gain imparted to the probe tone is reduced. In general, the cross gain modulation causes the gain to be overestimated at shorter wavelengths and underestimated at longer wavelengths, with the least distortion occurring in the middle of the bandwidth.
The effects of cross modulation can be substantially avoided if the probe tone does not serve as a time varying pump-source. That is, cross gain modulation is reduced if the number of photons supplied by the probe tone remains constant over time, or, in other words, if the power level of the probe tone is maintained at a constant level within a limited bandwidth. In this case the effects of Raman gain and the amplifier time dynamics remain substantially constant and can be eliminated when determining the gain profile. Accordingly, the gain profile can be more accurately determined by employing a probe tone that does not undergo amplitude modulation.
The present invention provides a method and apparatus for determining the gain profile of a path through an optical amplifier disposed in a first optical transmission path of an optical transmission system supporting bi-directional communication between first and second terminals along the first optical transmission path and a second optical transmission path. The method begins by generating an optical tone at a given wavelength within the bandwidth of the optical amplifier. The wavelength of the optical tone is modulated about the given wavelength. The modulated optical tone is transmitted from the first terminal along the first optical transmission path and through the optical amplifier. A portion of the modulated optical tone is received after it traverses the optical amplifier and FM to AM conversion is performed on the received portion to form an amplitude modulated optical tone. The amplitude modulated optical tone is transmitted back to the first terminal along the second transmission path via an optical loop-back path. Finally, the process is repeated for a plurality of different values of the given wavelength.
In one embodiment of the invention, the amplitude modulated optical tone is converted to an alternating electrical bias that is applied to a pump source pumping the optical amplifier. Additionally, the wavelength modulating step may be performed over a bandwidth comparable to the spacing between adjacent carrier wavelengths of a wavelength division multiplexed signal. The wavelength modulation may be performed at a repetition rate that is translated into an AM modulation having a frequency higher than the repetition rate.
In yet another embodiment of the invention, the FM to AM conversion is performed by an interference filter. Furthermore, the step of performing FM to AM conversion may include the step of translating each cycle of the FM modulation to at least two cycles of AM modulation.